Optical lenses are not only used as spectacles, but also used in a variety of situations, for example, as optical systems of various cameras such as cameras, film-integrated cameras and video cameras. Examples of important physical properties as such lens materials include a refractive index (nD) and an Abbe's number (ν). For optical design of an optical unit, use of a material with a high refractive index can realize a lens element that has a surface with a smaller curvature, which has advantages of decreasing aberration caused on this surface, decreasing the number of lenses, reducing eccentric sensitivity of the lens, and allowing reduction of size and weight of the lens system by decreasing the lens thickness.
Moreover, for optical design of an optical unit, combinational use of multiple lenses having different Abbe's numbers from each other is known to correct chromatic aberration. For example, a lens made of an alicyclic polyolefin resin having an Abbe's number ν of 45-60 and a lens made of a polycarbonate (nD=1.586, ν=30) resin composed of bisphenol A having a low Abbe's number can be combined to correct chromatic aberration.
As lens materials, optical glass and optical transparent resins are widely used. Optical transparent resins have advantages in that they allow production of aspherical lenses by injection molding and in that they allow mass production. Injection molding is a technique in which plastic is softened by heating, forced with injection pressure to fill in a mold to be molded, and then the molded body is taken out after cooling the resin.
Although the fluidity of the resin can be further enhanced by increasing the temperature for softening the resin, there is restriction on the softening temperature because of tendency of decomposition and coloring of the resin. In addition, while the mold temperature is kept constant in most molding machines, the upper limit of the mold temperature is limited to about 150° C. since pressurized water is used as a heat medium in a general mold temperature regulating machine. Accordingly, if such machine is used to produce a product with high surface accuracy, the upper limit of the glass-transition temperature of the resin that can be used is as high as about 160° C.
While a polycarbonate resin made of bisphenol A has been is widely used for optical lens applications, further enhancement of the refractive index of optical lenses is required due to expansion of the applications of optical lenses. Moreover, application of a polycarbonate resin made of bisphenol A has been limited because of the weakness of large birefringence. Therefore, development of an optical lens resin that has both high refractive index and low birefringence has extensively been conducted.
In order to enhance physical properties of a bisphenol A-type polycarbonate resin, copolymerization with other type of polycarbonate resin has been conducted. In particular, Patent Document 1 discloses that copolymerization with a structural unit represented by Formula (1) enhances the refractive index.

Patent Document 2 discloses a copolymer of a polycarbonate resin containing a structural unit having a fluorene structure and bisphenol A. The structural unit disclosed in this document is different from the structural unit represented by Formula (1).
Furthermore, as resins having a high refractive index, Patent Document 3 discloses copolymers in which bisphenol A-type polycarbonate or an aromatic polycarbonate resin is replaced with Formula (2). It is, however, described that although such resin composition has a higher refractive index, its glass-transition point exceeds 160° C.

Next, birefringence will be described. While polycarbonate resins made of bisphenol A are widely used for optical lens application, there is a limit to its application because of the weakness of large birefringence. In particular, in recent applications to cell-phone cameras and digital cameras, a camera lens with high imaging performance and lower birefringence has been required along with the increase in the resolution owing to increased number of pixels.
An example of a method for realizing low birefringence of a resin material includes a technique in which compositions having positive and negative birefringences, i.e., opposite signs, are used to cancel birefringences of each other (Patent Document 1). The positive or negative sign of birefringence is determined by difference between the polarizability in the polymer main chain direction and the polarizability in the polymer side chain direction. For example, a polycarbonate resin made of bisphenol A in which polarizability in the polymer main chain direction is greater than the polarizability in the polymer side chain direction has positive birefringence whereas a polycarbonate resin made of bisphenol having a fluorene structure whose polarizability in the polymer side chain direction is greater than the polarizability in the polymer main chain direction has negative birefringence. Therefore, low refractive index has been realized with a copolymer having a combination of structural units with such opposite signs of birefringences.
Meanwhile, polymers having a 1,1′-binaphthalene structure are described in Patent Documents 4-7. Specifically, Patent Documents 4 and 5 disclose polycarbonate resins having a 1,1′-binaphthalene structure, which do not have a structural unit represented by Formula (3) below. Patent Documents 6 and 7 describe polymers containing a structural unit represented by Formula (3) but they are polyester carbonate resins and not polycarbonate resins.
(in Formula (3), X represents an alkylene group with a carbon number of 1-4).
As describe above, a polycarbonate resin composition and an optical molded body that have a high refractive index and fluidity suitable for molding, that show low birefringence and that hardly cause optical distortion have not yet been provided.